Saw chain



May 18, 1965 R. R. CARLTON 3,183,948

I saw CHAIN Original Filed Oct. 12. 1959 4 Sheets-Sheet 1 INVENTOR.

RAYMOND R. CARLTON BY BUCKHORN, CHEATHAM G BLORE ATTORNEYS y 1955 R. R.CARLTON 3,183,948

SAW CHAIN Original Filed Oct. 12, 1959 4 Sheets-Sheet 2 I IN V EN TOR.

RAYMOND R. CARLTON 62 BUCKHORN, CHEATHAM 8 BLORE ATTORNEYS y 1965 R. R.CARLTON 3,183,948

I SAW CHAIN Original Filed Oct. 12, 1959 4 Sheets-Sheet 3 IN V EN TOR.RAYMOND R. CARLTON F/g. /2 F/g. /3 y BUCKHORN, CHEATHAM a BLOREATTORNEYS y 8, 1965 R. R. CARLTON 3,183,948

SAW CHAIN Original Filed Oct. 12, 1959 4 Sheets-Sheet 4 RA YMOND R. CARLro/v m/vewrom BY Buck/10R, BLORE, KLAROU/ST a SPAR/(MAN ATTORNEYS UnitedStates Patent 3,183,948 SAW CHAIN Raymond R. Carlton, Portland, 0reg.,assignor to Ornarir Industries, Inc, Portland, 0reg., a corporation ofOregon Original application Oct. 12, 1959, Ser. No. 845,788, now PatentNo. 3,040,602, dated June 26, 1962. Divided and this application May 2,1962, Ser. No. 191,822

3 Claims. (Cl. 143-135) This invention relates to a saw chain sharpenerand more particularly to a saw chain capable of being sharpened by themethod and with the general type of sharpener herein disclosed, whilethe chain is mounted upon a chain saw bar and being driven by a chainsaw sprocket. This application is a division of my copendingapplication, Serial No. 845,788 filed October 12, 1959, now Patent3,040,602,,issued June 26, 1962.

Mechanisms for use in sharpening a saw chain while in position upon achain saw and being driven by the chain saw sprocket have been suggestin the prior art. Such mechanisms have taken the form of saw sharpeningdevices temporarily mounted upon the nose end of the saw bar, which isthe end remote from the sprocket. Such devices have included sharpeningelements brought into contact with the cutter elements of the cutterlinks of scratcher type chain as such cutter links are moved around thenose of the cutter bar. The surfaces of the cutter elements thuscontacting the sharpening element are those most remote from the saw barwhich surfaces will be herein referred to as the outer surfaces of thecutter links.

By positioning the cutter elements of the cutter links rearwardly of themidpoint of lines joining the centers of the rivets or other pivots ofthe respective cutter links, it has been possible to sharpen scratchertype cutter elements and produce a type of relief on the cuttingelements which is effective for cutting with the portion of the chaintraveling along a side edge of the saw bar. This type of cutting isfrequently called slicing. The outer surfaces thus produced are,however, concentric with the curved nose portion of the saw bar. Theyhave no relief at all so far as the operation with a chain saw known asstabbing or boring is concerned. Such operation has become anestablished and important method of using a chain saw and consists ofpushing the nose end of the chain saw endwise into a tree, log or otherobject being cut so that the cutting is actually accomplished by thecutter elements which are being carried around the nose end of the bar.Unless such cutting elements are provided with properly relieved outersurfaces, the boring operation is inefficient and difficult. Thusexcessive force is required to force the nose end of the chain saw intothe wood being cut. Mounting the sharpening mechanism upon the nose endof the cutter bar has also required that the chain saw be taken out ofservice while the sharpening mechanism is being attached and detached aswell as during the time a sharpeningoperation is being actually carriedout. This has restricted the utility of prior art sharpeni'ng devicesintended for use while the saw chain is mount: ted upon a chain saw andis being driven by the chain saw sprocket.

, The difficulties above. discussed are. eliminated by mounting asharpening device directly upon thebody of the chain saw adjacent thechain saw sprocket soasto remove metal-from the outer surfaces of thecutter links. Such sharpening device can be left in place upon the sawat all times and, if desired, can actually be employed to sharpen thesaw chain upon the chainsawwhile thesaw is being employed in a cuttingoperation. In anyevent,

3,183,948 Patented May 1 8, 1965 the sharpening device need not beremoved during operation of the saw but may be allowed to remainattached to the saw at all times so that it is not necessary to take thesaw out of service while attaching and removing a sharpening mechanism.Also by employing a chain saw sprocket of substantially smaller radiusthan the radius of curvature of the nose end of the saw bar, properrelief of the outer surfaces of the cutter elements of the saw chain canbe produced so that eflicient boring with the sharpened saw chain aswell as slicing can be accomplished.

The sharpening element of the saw chain sharpener can be shaped so as toeffectively sharpen nearly any type of scratcher saw chain which has thecutting elements thereof rearwardly of the midpoints of the respectivecutter links as discussed above. The sharpening device is, however,particularly effective for sharpening a new type of chain disclosedherein in which each cutter link whether it be of a type having a sideslitter cutter element or of a type having a raker cutter elementthereon or any other type, is also provided with a depth gauge which hasan outer kerf bottom contacting surface spaced forwardly of suchmidpoint of the respective cutter link a greater distance than thecutting portion of the cutter element of such link is spaced rearwardlyfrom such midpoint. The outer surfaces of such depth gauges are thusgiven proper relief not only for slicing operations of the chain saw butalso for boring operations. The relief of the outer surfaces of suchdepth gauges as well as the relief of the outer surfaces of the cutterelements of a properly constructed chain is of such uniformitythroughout the length of the chain as to provide a smooth operation andrapidity of cutting of a greater order than possible with prior sawchains.

An object of the invention is to provide a new and improved type of sawchain particularly adapted for being sharpened by the method and withthe sharpening devices shown and described in the present application.

I Other objects and advantages of the invention will appear in thefollowing description of the invention including the structures shown inthe attached drawing of which:

FIG. 1 is a somewhat diagrammatic side elevation of a chain saw showinga sharpening device adapted to sharpen the saw chain of the presentinvention mounted thereon;

FIG. 2 is a fragmentary side elevation showing a portion ofthe'structure of FIG. 1, including the saw chain sharpening devicethereof on a greatly enlarged scale and with parts of the chain sawsprocket broken away;

FIG. 3 is a fragmentary horizontal section through the structure of FIG.2 showing the saw chain sharpening device in top plan;

FIG. 4 is an isometric view of the saw chain sharpening device on astill larger scale;

FIG. 4a is a fragmentary view on an enlarged scale and partly invertical section of a portion of the sharpening element of the deviceshown in FIGURE .4;

FIG. 5 is a side elevation of the saw chain sharpening device withcertain parts in a different position than in FIGS. 2 and 4; FIG. 6 isan isometric exploded view of the saw chain sharpening device of FIGS. lto 5;

FIG. 7 is a fragmentary side elevation showing a modified manner ofmounting the saw chain sharpening device of FIGS.; 1 to 6 upon a chainsaw;

FIG. 8 is a view similar to FIG. 1 showing a modified saw chainsharpening device;

FIG. 9 isa somewhat diagrammatic horizontal section through the chainsaw of FIG. 8 on line 9,9 of FIG. 8;

FIG. 10 is a side elevation of a saw chain of the presareaaee outinvention which saw chain is particularly adapted for sharpening by themethod and by the sharpening devices disclosed in the presentapplication;

FIG. 11 is a top plan view of the saw chain of FIG.

FIG. 12 is a perspective view of a side slitter cutter link of the sawchain of FIGS. 10 and 11;

FIG. 13 is a view similar to FIG. 11 of a raker cutter link of the sawchain of FIGS. 10 and 11; and

FIG. 14 is an enlarged side view of the raker cutter link shown in FIGS.10 and 13.

Referring more particularly to the drawing, a chain saw is shown in FIG.1 as having a saw chain sharpening device 22 mounted upon the motorhousing or body 24 of the saw in position to sharpen a saw chain 26mounted upon a saw bar 28 secured to and extending from thebody 24 ofthe saw. The chain 26 is also mounted upon and driven by a chain sawsprocket 36 which in turn is mounted upon a sprocket drive shaft 32driven by the saw chain motor (not shown). The sprocket 30 has a smallerpitch radious than the pitch radius of the nose end 34 of the saw bar 23and the sharpening device 22 has a sharpening element 36 positioned forcontact with the outer surfaces of the various cutter links 38, 40, 42and 44 as they are carried around the periphery of the sprocket 3t Thedetails of the sharpening device 22 are shown in FIGS. 2 to 6 andinclude a base block 46 pivotally secured adjacent its lower rightcorner in FIGS. 1 and 2 to the body 24 of the chain saw by means of ascrew 48 having an enlarged head received in a socket 50 (FIG. 6) in thebase block 46 and having a shank screw threaded into the body of thechain saw. The base block 46 also' has an arcuate slot 52.concentricwith the axis of the screw 48 extending through the body of the baseblock adjacent its end opposite the screw 48. Such slot 52 receives theshank of a clamping screw 54 having an enlarged knurled head, the end ofthe shank of the screw also being screw threaded into the body 24 of thechain saw. It will be apparent that the base block 46 of the 4 groove 82in the shank of the screw 74. The shank of the screw is threaded into avertically extending bore in the slide member 62. By rotating the screw74 by its knurled and slotted headfld, the slide member can be movedfrom the upper position shown in FIG. 4 toward the lower position shownin FIG. 5.

The axis of the sharpening element 36 is inclined to the direction ofsliding motion of the slide member 62 provided by the dovetail portion66 of the slide member so that the top end of the sharpening element isspaced farther from such dovetail portion than the lower end of suchsharpening element. The axis of the sharpening 7 element is also in thecentral plane of the saw chain 26 sharpening device 22 can be pivotedabout the screw 48 between the operative position of the sharpeningdevice shown infull lines in FIG. 2, and the inoperative position ofsuch device shown in dash-dot lines in such figure and that the baseblock 46 can be clamped in either position by the clamping screw 54.

The sharpening element 36 is of cylindrical shape having an axial bore56 (FIG. 6) therein and an outer cylindrical surface provided with aplurality of helically extending metal cutting teeth 58 (FIG. 4)preferably of the form shown in such figure with properly relieveddownwardly directed cutting edges 60' Such sharpening element ispreferably made of a hard alloy capable of remaining sharp whileemployed to remove metal from the cutter links of a saw chain. Thesharpening element is supported between the yoke portions of a yokeshaped slide member62 and is journaled for rotating in such slide member62 about the axis of a screw 64 (FIG. 6) extending through the yokeportions of the slide member and having one end screw threaded into oneof such yoke members. The slide member 62 is mounted for verticalsliding motion along the edge of the base ,block 46 by means of adovetail element 66 on the slide member 62 The slide member can beadjusted vertically of the base block 46'by an adjusting screw 74 havinga shank extending through a bore in a top plate 76 secured to the baseblock 46 and removable guiding element '70 by the screws 78. The screw74 is held against axial movement relative to the plate 76 by a pin 8%extending laterally .thr'ougha bore 81 in the plate 76 and engaging in aand sprocket 3t) therefor and it will be apparent from FIG. 2 thatdownward adjustment of the slide member 62 by the screw 86, when thesharpening device is in the operative position shown in full lines inFIG. 2, will advance the sharpening element toward the saw chain 26 onthe sprocket 31).

The operation of the saw sharpening device of FIGS. 1 to 6 should beapparent from the above description thereof. With the sharpening deviceclamped in the position shown in FIG. 1 and in full lines in FIG. 2 bythe screw 54, the saw is started and the saw chain 26 on the sprocket 30and saw bar is driven by the sprocket so as to move the saw chain cutterlinks 38, 4t), 42 and 44 in a clockwise direction around the peripheryof the sprocket. The screw 86 is adjusted to cause the sharpeningelement 36 to engage the outer surfaces of the cutter links of ascratcher type chain to remove metal therefrom. Since the sharpeningelement is convex toward the saw chain, the cutter portions of the rakercutter links are cut or ground lower than the cutter portions of sideslitter cutting links. This is true because such portions of rakercutter links are nearer the central plane of the chain than those ofside slitter cutter links. Also since the cutter teeth on the sharpeningelement are of helical form, the sharpening element revolves on thescrew 64 to constantly present new cutting edges and distribute the wearcircumferentially around such element. After a sharpening operation, thesharpening device 22 can be moved to the inoperative position shown indotted lines in FIG. 2 until another sharpening operation is needed.

An improved type of chain particularly adapted for being sharpened bythe method and sharpening devices described above is shown in FIGS. 10to 13 inclusive. Such chain is shown in FIGS. 10 and 11 as having a leftslitter cutter link 38 followed by a right side slitter cutter link 46and a left raker cutter link 42 followed by a right raker cutter link44. The order of such links is different from that shown in FIG. 2 butthe order of such links may be varied. In general a pair of side slittercutter links made up of right and left side slitter cutter links will befollowed by a pair of raker cutter links made up of right and left rakercutter links and the latter pair will again be followed by a pair ofside slitter cutter links, etc. Also, in general, alternate cutterlink-s will be right and left cutter links.

A left side slitter link is shown in FIG. 12 as having an outwardlyprojecting cutter element .88 which is positioned rearwardly ofthemidpoint 90 of a line 92 joining the centers of the rivet holes 94 ofthe cutter link 38 so that the point .90 is the midpoint of a linejoining the centers of the pivots of such link. The cutterelement 88 isinclined laterally of the chain in both a direction outwardly of thechain and forwardly thereof, as shown most clearly in FIG. 1 1 and has aleading edge portion 96 beveled to provide a sharpened forwardlydirected edge at the lateral extremity of. the cutter element, whichedge is inclined forwardly. and laterally of the cutter element. Theengagement of the sharpening element 36 of FIGS. 1 t0 6 with the cuttingelement 88 while the cutter link 38 is being carried around the sprocket39 will produce an arcuate beveled surface 98 and a cutting edge whichhas proper relief rearwardly of the leading edge 96.

The cutter link 38 also has an outwardly projecting depth gauge 100thereon having its trailing edge positioned forwardly of the midpoint 90of the line 92 a greater distance than the leading edge 96 of the cutterelement 88 is positioned rearwardly of such midpoint. This means thatthe outer kerf bottom engaging surface 102 of depth gauge 100 ispositioned a greater distance forwardly of the midpoint 90 than thecutting portion of the cutting element 88 is positioned rearwardly ofsuch midpoint. This further means that the surface 102 is nearer theline 92 than the cutting portion of the cutter element. Such cuttingportion is the leading outer corner of such cutting element. Thisprovides the depth gauge 100 with proper relief for cutting operationsincluding both slicing and boring operations. The cutter link 40 isallochiral to the cutter link 38 and will not be further described. 7

The left raker cutter link 42 is shown most clearly in FIG. 13 andincludes an outwardly projecting raker cutter element 104 which islaterally thickened by having an extending portion 106 bent back uponitself and spot Welded to the body of the raker element. The rakerelement is inclined slightly laterally in both an outward and forwarddirection and has a leading edge portion 107 positioned rearwardly ofthe midpoint 90 of the link 42. The edge portion 107 is inclinedforwardly as well as laterally. The raker cutter element 104 has metalremoved from its outer surface 108 by the sharpening element of FIGS. 1to 6 during sharpening of the chain to provide a sharpened cutting edge110. The depth gauge 100 of the link 42 may be identical with the depthgauge 100 of the link 38 and may occupy the same relative position withrespect to its associated cutting element so that its outer surface 102is nearer the line 92 than the cutting edge 110 to provide proper relieffor such depth gauge. The right raker cutter link 44 is allochiral tothe link 42 and will not be further described.

It is apparent that the outer end surfaces 98 and 108 of the cutterelements 88 and 104, respectively, and the outer end surfaces 102 of thedepth gauges 100 will be arcuate convex surfaces and that these arcuatesurfaces of both the cutter and depth gauge elements have the sameradius of curvature and are concentric about a common axis parallel tothe pivot axes of the link.

The relationship is clearly shown for raker link 42 in FIG. 14. Therethe outer end surface 108 of the cutter element 104 and the outer endsurface 102 of the depth gauge 100 are shown to be arcuate convexsurfaces having the same radius R, and are concentric about an axisthrough point P. This axis lies in a plane A passing through themidpoint 90 of the line 92 joining the pivot axes. Plane A isperpendicular to line 92. As can be seen, the trailing edge 109 of thedepth gauge 100 is positioned forwardly of midpoint 90 a greaterdistance than the cutting edge 110 is positioned rearwardly of midpoint90. Thus, the edge 109 is closer to line 92 than is the edge 110 by thedistance d measured from plane B, where d is the amount of clearancenecessary for proper cutting operations. It is thus apparent that due tothe particular configuration of the links of the present invention, thisclearance d is correctly maintained even as the chain is sharpened whilein position upon the chain saw. Since the radius of curvature of thenose of the saw bar is greater than radius R, the distance d will insureproper clearance for boring as Well as slicing operations.

It should be noted, however, that the outermost edge of the cutterelement 88 on the slitter cutter links 38 and 40 is curved to a radiusgreater than the depth gauges 100 due to the bevel on the upper surface98 thereof, it being noted that the radius of curvature of the depthgauge 1110 is more nearly equalto that of the innermost edge of surface98. This is clearly shown in FIG. 10. Thus, whereas in the case of theraker link 42 shown in FIG. 14 the outer surfaces 108 and 102 of thecutter element 104 and depth gauge 100, respectively, have the sameradius of curvature R, in the case of the slit-ter links 38 and 40, theradii of curvature of the outer surfaces 98 and 102 are onlysubstantially the same, due to the bevel on the surface 98 as abovedescribed.

Scratcher chains have been known to be smooth running, rapid cuttingchains when properly sharpened and the main reason they have not beenmore generally employed is that proper sharpening has been difiicult andtime consuming even for an expert when compared to other more generallyemployed chains. With the present chain having depth gauges on allcutter links and precise sharpening such as provided by the method anddevices of the present invention, a new order of rapid cutting andsmooth operation is accomplished with a chain which can be sharpened ina matter of seconds without removing the chain saw from service.

With chain saws which cannot be modified to install the sharpeningdevice of FIGS. 1 to 6 upon the body of the chain saw, it is possible totemporarily install the sharpening device 22 upon a support plate 112shown in FIG. 7 and then secure such plate to the nose end of a suitablymodified chain saw bar 113. The sharpening device 22 can be employed tosharpen cutter links 38, 42 and 44 as they are carried around the noseend of the bar. This merely illustrates the versatility of thesharpening device 22 but does not accomplish all of the results of themethod discussed above. The employment of the sharpening deviceasillustra-ted in FIG. 7 will sharpen a chain of the type illustrated orany of the types discussed above adequately for slicing operations ofthe chain saw but not for efficient boring operations, since properrelief is not provided for boring.

In any of the uses of the sharpening devices 22, the sharpening elementmay, for example, be of tungsten carbide, tantalum carbide or titaniumcarbide, or any of the other hard carbides or other hard materialsemployed as cutting elements for removing metal such as steel from metalsurfaces. A tungsten carbide metal removing sharpening element formedwith a plurality of helical cutting teeth, such as shown in FIG. 4, hasbeen found particularly effective but other metal removing elements,such as suitable soft metals with diamonds or other hard materialsembedded therein, can be employed.

A modified sharpening device 114 for carrying out the method of thepresent invention is shown in FIGS. 8 and 9. In such figures a saw bar28 and sprocket 30 support and drive a saw chain 26 and all of suchelements may be the same as the corresponding elements described withreference to FIGS. 1 to 6 and 10 to 13. The chain saw has a modifiedbody portion 116 in the form of a casing in which the main drive shaft118 of the saw is journaled, such shaft having the sprocket 30 mountedupon one of its ends which is external to the casing 116.

The sharpening device 114 includes another shaft 120 journaled in abearing block 121 slidable in the casing 116 toward and away from theshaft 118. The shaft 120 is driven from the shaft 118 by a belt 122 onpulleys 124 and 126 on the shafts 118 and 120, respectively, the pulley126 being any suitable or known type of spring expansible pulley so thatthe shaft 120 can be moved toward and away from the shaft 26 while beingdriven by the belt 122. Movement of the shaft toward and away from theshaft 118 can be accomplished by sliding the bearing block 121 in thecasing 116 by means of a screw 127 passing through the end wall of thecasing 116 and having its shank screw threaded into the bearing block121. The screw is rota-table in the casing 116 but is held against axialmovement therein by a pin 128 engaging in a groove in the shank of thescrew 127. It is apparent that turning of the adjusting screw willadvance the shaft 120 toward the shaft 118 or retract it depending uponthe direction the screw is turned. 7

The shaft 120 has an end projecting through a slot 129 in a side wall ofthe casing 116 and has a metal removing member in the form of a grindingwheel 130 mounted on such end exteriorly of such casing. The centralplane of the grinding wheel 130 is in the central plane of the sprocket30 and chain 26 and has its outer periphery shaped to provide bevelededges 132 for properly grind ing the cutter elements of chains of thetypes above described. Such shape of the outer periphery of the grindingwheel may be maintained by a grinding wheel dresser 134 which may be ofany suitable type, such as a conventional pressure dresser. The dresser134 is rotatively mounted upon the end of a lever 136 pivoted to thecasing 116 at 138 so that the dresser 134 can be manually held againstthe periphery of the grinding wheel 130.

It will be apparent that the shaped outer periphery of the grindingwheel 130 may be brought into engagement with the outer surfaces of thecutter elements of the cutter links of the saw chain 26 while suchcutter links are being carried around the sprocket 30 and while thewheel 130 is being rapidly rotated to remove metal from such outersurfaces. The result is to sharpen the saw chain and provide properlyrelieved outer surfaces on the cutter elements and any depth gauges onthe chain of the same type described above with reference to thesharpening device of FIGS. 1 to 6. Again the saw chain can be sharpenedwhile in position upon the saw and without taking the saw out of serviceto provide a properly sharpened smooth running, fast cutting saw chain.

I claim:

1. A saw chain cutter link for use in a saw chain which can be sharpenedwhile mounted upon a chain saw and While being positioned on and drivenby the chain saw sprocket, comprising:

a linkbody having forward and rear ends and having means definingparallel pivot axes spaced from each other longitudinally of said link,

a cutter element projecting outwardly from the rear of said body andhaving on its outer portion a forwardly projecting cutting edgepositioned rearwardly of the midpoint of the line joining said pivotaxes,

V a depth gauge element projecting outwardly from the front of said bodyin the same direction as said cutter element and a lesser distance fromsaid line joining said pivot axes than said cutter element,

said depth gauge element being spaced from said cutter element forwardlyof said body and having its outer trailing edge positioned forwardly ofsaid midpoint a greater distance than said cutting edge is positionedrearwardly of said midpoint,

each of said cutter and depth gauge elements terminating at its outerend in a convex arcuate surface,

the arcuate surfaces of both said elements having substantially the sameradius of curvature and being concentric about a common axis parallel tosaid pivot axes and in a plane through said midpoint, said plane beingperpendicular to said line joining said pivot axes.

2. A saw chain cutter link as described in claim 1, in which said cutterelement is inclined laterally and has a leading outwardly extending edgeinclined forwardly and laterally of said cutter element.

3. A saw chain cutter link as described in claim 2, in which theoutermost edge of said arcuate surface of said cutter element has agreater radius of curvature than does said arcuate surface of said depthgauge element.

References Cited by the Examiner UNITED STATES PATENTS 2,832,180 4/58Carlton 143135 X 2,832,380 4/58 Crowe. 3,040,602 6/62 Carlton.

OTHER REFERENCES Stihl: German application 1,098,186, printed Jan. 26,1961 (KL. 38a 11), (1 sht. dwg., 2 pp. spec.).

WILLIAM DYER, IR., Primary Examiner.

DONALD R. SCHRAN, Examiner.

1. A SAW CHAIN CUTTER LINK FOR USE IN A SAW CHAIN WHICH CAN BE SHARPENEDWHILE MOUNTED UPON A CHAIN SAW AND WHILE BEING POSITIONED ON AND DRIVENBY THE CHAIN SAW SPROCKET, COMPRISING: A LINK BODY HAVING FORWARD ANDREAR ENDS AND HAVING MEANS DEFINING PARALLEL PIVOT AXES SPACED FROM EACHOTHER LONGITUDINALLY OF SAID LINK, A CUTTER ELEMENT PROJECTING OUTWARDLYFROM THE REAR OF SAID BODY AND HAVING ON ITS OUTER PORTION A FORWARDLYPROJECTING CUTTING EDGE POSITIONED REARWARDLY OF THE MIDPOINT OF THELINE JOINING SAID PIVOT AXES, A DEPTH GAUGE ELEMENT PROJECTING OUTWARDLYFROM THE FRONT OF SAID BODY IN THE SAME DIRECTION AS SAID CUTTER ELEMENTAND A LESSER DISTANCE FROM SAID LINE JOINING SAID PIVOT AXES THAN SAIDCUTTER ELEMENT, SAID DEPTH GAUGE ELEMENT BEING SPACED FROM SAIDCUTTERELEMENT FORWARDLY OF SAID BODY AND HAVING ITS OUTER TRAILING EDGEPOSITIONED FORWARDLY OF SAID MIDPOINT A GREATER DISTANCE THAN SAIDCUTTING EDGE IS POSITIONED REARWARDLY OF SAID MIDPOINT, EACH OF SAIDCUTTER AND DEPTH GAUGE ELEMENTS TERMINATING AT ITS OUTER END IN A CONVEXARCUATE SURFACE, THE ARCUATE SURFAFCES OF BOTH SAID ELEMENTS HAVINGSUBSTANTIALLY THE SAME RADIUS OF CURVATURE AND BEING CONCENTRIC ABOUT ACOMMON AXIS PARALLEL TO SAID PIVOT AXES AND IN A PLANE THROUGH SAIDMIDPOINT, SAID PLANE BEING PERPENDICULAR TO SAID LINE JOINING SAID PIVOTAXES.